What is the difference between Polyjet and Fused Deposition Modelling (FDM)?
For those that do not recognise Polyjet, this is the process name behind the Objet line of printers. Today both processes are supplied by Stratasys and are not likely contenders to put against each other. That said there are some unexpected similarities!
Process choice, as is so often the case, hangs on your objectives.
If we start with the manufacturing process, there are a number of differences between these two additive processes…
Different additive process
Whilst FDM extrudes a continuous ‘string’ of thermoplastic to build up a part layer by layer, Polyjet uses multiple nozzles to ‘spray’ or ‘jet’ UV sensitive material that is then cured layer by layer by a UV light.
They both use support structures built in a similar way to that of the main part; FDM support is extruded whilst Polyjet supports are ‘spray’ deposited.
The design requirements differ for both processes and this is also true for the support structure – we’ll come back to that later.
Different materials
Fundamentally the materials are different. FDM uses thermoplastic materials whilst Polyjet uses thermosetting materials.
FDM has some of the common prototyping and production materials such as ABS, ASA, PC-ABS etc whilst Polyjet materials are believed to be acrylic formulations, quoted as ABS-like. Polyjet has the additional benefit of being able to print with elastomeric materials.
FDM systems can also accept specialist materials such as electrostatic dissipative and FST (Flame, smoke and Toxicity) rated materials. Polyjet on the other hand allows multiple materials, colours and shore hardness to be used within the same model (creating overmoulded parts for example).
Whilst dual material hobbyist FDM systems are available, we wouldn’t recommend these for high quality prototypes (just yet) and this isn’t feasible with high-end systems (unless you can use the support material as your 2nd material that is).
New FDM materials are continually emerging such as those with specific chemical resistances and materials suitable for sterilization, as are new Polyjet materials such as biocompatible and dental materials.
Different design requirements
Polyjet requires support structure for even the smallest overhang whilst FDM can build moderate overhangs (depending on the geometry) without support structures.
Whilst both can tolerate internal hollow features, the support structure still needs to be removed. With Polyjet this is a combination of waterjet and manual abrasion, whilst there is a soluble support structure available for some of the FDM materials – but this can be time consuming and isn’t always a straightforward process.
The minimum layer thickness and feature size can be much smaller with Polyjet.
Using FDM standard ‘infill’ patterns can reduce build time and costs and parts are frequently not solid.
File format requirements
Whilst standard stl files can be used with both systems, this can become more complicated when pushing the potential of Polyjet.
For example for parts with textures and multiple colours/materials this information needs to be given to the machine and we could write another article on this topic alone!
Knowledge of these can help you take advantage of the manufacturing process for example – you can use the automatic infill feature of FDM to make your part more light weight and reduce material usage and costs.
So to sum up, the two different processes each have their own advantages. If you have a niche material requirement or you’re looking for a functional prototype then FDM may be your best option. Likewise if you want to have a part with rubber-like and rigid materials in the same part – or a lifelike prototype with colours/textures and fine detail to match your final product, then Polyjet might be your best option.
Either way there are some unusual design rules and file format requirements for these processes so get in touch early on in your project and we can help you save design time and help you to make the most out of your prototype.
